Background
In 2017 Yolo County received a grant from the California Department of Water Resources Small Community Flood Risk Reduction Program to complete a feasibility study with a primary goal of reducing flood risk to the community of Yolo. The scope of the study is to identify preferred structural and non-structural elements, compare implementation costs and schedules and identify local funding requirements to assess options which will reduce the flood risk to the community of Yolo. The study goal is to achieve a 100-year level of flood protection for Yolo through potential repairs and improvements to the levee system, and if necessary, adjoining highway embankments immediately west of the community.
Per California Water Code 8361, on behalf of the State, it is the California Department of Water Resources (DWR’s) responsibility to maintain and operate certain portions of the Sacramento River Flood Control Project that specifically includes the levees directly adjacent to Yolo along both the left (north) and right (south) banks of Cache Creek.
Summary of Draft Study
The community of Yolo (formerly known as Cacheville) is located adjacent to the left (north) bank of the Cache Creek levee directly downstream of where Interstate 5 crosses Cache Creek, approximately 5 miles northwest of Woodland, California. The Cache Creek levees were improved by Unites States Army Corps of Engineers (USACE) in 1958 for to convey a target flow of 30,000 cfs, which corresponds to a 10-percent annual chance flood with the anticipation of Wilson Valley Dam and Reservoir for flood control upstream, which was never constructed. Due to the lower design flow, the 100-year peak flow of 63,680 cfs could over top at several locations upstream of the town of Yolo. As a result, community of Yolo hasn’t experienced flooding yet. However, the community of Yolo has come very close to flooding in recent years with the left (north) bank levees of Cache Creek overtopping in 1997, 2006 and 2019, occurring both upstream and downstream of Yolo.
The purpose of this study is to identify structural and non-structural flood risk reduction actions and multi-benefit opportunities, compare implementation costs and schedules to identify a preferred alternative that will reduce the flood risk to Yolo along with identifying local funding requirements while sustaining adjoining agriculture and the regional economy. The study goal is to achieve a 100-year level of flood protection for Yolo through potential repairs and improvements to the levee system, and if necessary, adjoining highway embankments immediately west of the community.
To address the study goals, six project alternatives were formulated and evaluated:
- Vegetation clearing/Sediment removal/Dredging
- Construction of additional setback levees
- Construction of upstream flood control storage reservoir(s)
- Restore/Repair left (north) bank of Cache Creek to USACE 1957 Design Profile, including freeboard deficiencies and remediation of erosion concerns
- Levee improvements for left (north) bank of Cache Creek to convey a 100-year flood event
- Restore left (north) bank of Cache Creek levees to USACE 1957 Design Profile and levee improvement to convey a 100-year flood event
Preferred Alternative
The preferred alternative was identified as #6 - ‘Restore Left Bank of Cache Creek levees to USACE Design Profile and Levee Improvements to Pass 100-year Flow’. This alternative includes include 0 to 4 feet of levee raise above its current height adjoining the community of Yolo and the levee widened at is base by as much as 10 to 15 feet in certain locations, particularly along the downstream, easterly portion of the levee system. In addition to the levee raise, the half-mile left bank levee segment directly adjacent to the community of Yolo would be improved with an appropriate cutoff wall to address underseepage and potentially rock slope protection to address erosion.
The total cost for the preferred levee improvements is estimated to be approximately $16.7 million. Since the elements of the preferred alternative includes all the elements for ‘Restore Left Bank of Cache Creek to USACE 1957 Design Profile’ which is primarily the State’s responsibility, the study indicates that State should pay for approximately $9.7 million. The remaining $7 million could be cost-shared by the State, Yolo County, the community and other available funding sources.
Other Recommended Improvements
Several non-structural measures have been identified for their potential to reduce residual flood risk and can be implemented independent of or in combination with the preferred structural improvements. The most implementable for this region are voluntary flood proofing, voluntary elevation and changes to the National Flood Insurance Program (NFIP). The study includes a recommendation to conduct a Base Flood Elevation (BFE) study, to determine the elevation to which floodwater is anticipated to rise during the FEMA 100-yr. base flood, a regulatory requirement for the elevation, floodproofing or new construction of structures, which is currently not available for the town.
Potential Restoration Opportunities
In addition to these structural and non-structural measures, nine preliminary habitat restoration concepts and one groundwater recharge concept were narrowed through a screening process to four with the potential to be implementable in connection with the flood risk reduction alternatives identified in this study. These four concepts include the Cache Creek Non-Native Species Control Concept, the Working Waterways Projects Implementation Concept, the Cache Creek Gravel Pit Restoration Concept, and the Cache Creek Gravel Pit Recharge Basin Concept.
Potential Recreational Opportunities
The Study also evaluated recreational opportunities and identified the Cache Creek Parkway Plan as a long-term vision for recreational opportunities to optimize the recreational benefits for the town of Yolo. It is recommended that any flood system planning, and design improvements proposed within Yolo also consider the integration of the multi-benefit recreational opportunities identified in the Parkway Plan, particularly those east of CR 94B near the town of Yolo along with Class II bike path improvements between Yolo and Woodland.
Relationship with other Cache Creek Efforts
Channel Maintenance
As noted above, the Cache Creek levee system was designed in anticipation of an upstream reservoir (Wilson Dam and Reservoir) that would reduce peak flows on Cache Creek. The reservoir was never constructed, leaving the levees only able to convey a peak flow of roughly 30,000 cfs (estimated to be a 10-year flood event). Ergo, the levees are undersized and not designed to convey 100-year flood events. Channel maintenance, such as gravel bar removal (i.e. dredging) and removal of vegetation, is necessary to allow the channel to convey the target flows, but will not provide adequate capacity to meet 100-year flood protection.
Regulatory Constraints
As the Board is aware, the County has struggled to renew its Army Corps "Regional General Permit", which would allow for routine channel maintenance in Cache Creek, since 2011. Currently staff is working with our federal legislative advocates to advance the processing of the Corps permit.
Huff's Corner
It is worth noting that the County's Huff's Corner repair and rehabilitation project will address erosion issues under and along County Road 18, protecting that valuable infrastructure, and will restore the levee height to the original design standard, but it will not provide greater flood protection than what the levee system was designed to convey.
Staff Recommendation
Staff recommends that the Board receive the presentation on the preliminary alternatives for the flood risk reduction for the community of Yolo and provide feedback and direction as appropriate. |